"Last year I had an affair," Ensign, 51, said during a news conference in Nevada. "I violated the vows of my marriage. It is the worst thing I have ever done in my life. If there was ever anything in my life that I could take back, this would be it."
He did not name the female campaign aide involved in the affair, but described the woman and her husband as family friends.
"Our families were close," Ensign told reporters. "That closeness put me into situations which led to my inappropriate behavior. We caused deep pain to both families and for that I am sorry."
Ensign's wife, Darlene, also released a statement about the affair.
"Since we found out last year, we have worked through the situation and we have come to a reconciliation," she says.
Locally, Ensign is known for opening the first 24-hour animal hospital in Las Vegas after receiving his veterinary degree in 1985 from Colorado State University. In Washington and throughout the veterinary community, he is known for introducing several bills dealing with animal rights and safety, including the Animal Fighting Prohibition Enforcement Act of 2005, which aimed to stop animal fighting by making transportation of animals across state lines for the purpose of fighting a felony under federal law. He has been working for several years on anti-horse slaughter legislation.
Ensign no longer practices.
He was elected to the Senate in 2000 and has recently been tapped as a potential presidential hopeful in the 2012 election.
His title as chairman of the Republican Policy Committee put him in the No. 4 position in leadership. The committee coordinates the GOP's legislative efforts in the Senate.
Ensign told the press he has no intention of resigning.